1) Field of the Invention
The invention relates to generating a frequency distribution for dimensions of objects to be imaged, such as fibers used in pulp and paper processes.
2) Description of Related Art
Paper industry, for example, uses pulp processes in which the average length of fibers and the distribution of their length are often among the most essential parameters. Length distribution is usually expressed as proportional frequencies (weighted by length) of different length categories, and thus the values of the length categories can be interpreted as percentage shares of the entire pulp mass.
One method of measuring these parameters is to produce an image of a pulp suspension of low consistency, with the fibers in the suspension dispersed into water acting as a medium, and to aim at determining, by means of an automated image processing equipment, a representative amount of sample fibers from the pulp to establish the nature of the pulp. A similar method can be used to measure other objects than pulp as well.
However, this measurement involves problems. When objects to be imaged are in a space in which the position of each object is at least partly free and when the dimension of each object in the image depends on its position, the projected dimension of the object can be measured from the image. The projection distorts the frequency distribution of the dimensions of the objects and thus measurements carried out this way do not correspond to reality. Moreover, if the space that is being imaged is smaller than the space in which the objects are allowed to move, the margin of the image forms an area in which only the part of the object that is within the margin can be imaged, and thus it is not possible to determine the real dimension of the object. Further, in the middle of the image there may be discontinuities caused by impurities or cracks in the measurement optics, which will affect the visibility of the dimensions and their measurement.